There is a general need for invasive measurements of physiological variables. For example, when investigating cardiovascular diseases, it is strongly desired to obtain local measurements of pressure and flow in order to evaluate the condition of the subject under measurement. Therefore, methods and devices have been developed for disposing a miniature sensor at a location where the measurements should be performed, and for communicating with the miniature sensor.
An example of a known intracranial pressure monitor is known through U.S. Pat. No. 4,026,276, in which it is described an apparatus including a passive resonant circuit having a natural frequency influenced by ambient pressure. The local pressure is measured by observation of the frequency at which energy is absorbed from an imposed electromagnetic field located externally of the cranium.
In order to communicate a measured representation of the physiological variable, devices based on acoustical as well as electromechanical interaction have been developed. In both cases, the sensor comprises a resonance element, its resonance frequency being a function of the physiological variable to be determined. Energy is radiated towards the resonance element from an external transmitter of acoustical or electromagnetic waves, respectively. The frequency of the transmitted energy is swept over a pre-selected range, and is registered by a monitoring unit. During the frequency sweep the registering unit will detect the resonance frequency of the resonance element, since a drop of the monitored transmitted energy will occur at this frequency.
The example above of a device for invasive measurements of physiological variables is an example of a passive system, i.e. the sensor inside the body does not require a source of energy, such as a battery or electricity provided via electrical leads. For guiding a sensor to a specific point of measurement during investigating cardiovascular diseases it is known to mount a miniature sensor at the distal end of a guide wire or a catheter. The guide wire or the catheter is inserted into a blood vessel such as the femoral artery, and is guided by fluoroscopy to local sites within the cardiovascular system where improper functioning is suspected.
The development of miniature sensors, or micro-sensors, for a number of physiological variables, including pressure, flow, temperature etc., constitutes a historical medical technology landmark. However, the assembly of the sensor and the associated cables and connectors is difficult to perform in a cost-efficient manner due to the small physical dimensions, the required mechanical precision and uncompromisable demands on patient safety. More specifically, it is estimated that about one third of the cost, or more, of the total manufacturing cost for such devices are traceable to connectors and cables. As a consequence, devices performing these functions are still expensive, and the spread of their use is limited to areas of highest clinical priority. The cost aspect is further emphasized by the fact that devices for invasive procedures must be regarded as disposable items, due to the risk of transmitting infectious diseases. If the cost of cables and connectors could be minimized or even eliminated, large savings would be possible.
Another problem with passive sensors of the type disclosed in U.S. Pat. No. 4,026,276 is undesired electromagnetic coupling between the transmitter/receiver on the one hand, and the sensor on the other. This coupling is due to the fact that the power supply and the signal transmission are not functionally separated. A manifestation of this problem is that the output signal of the system is influenced by the position of the sensor, which obviously is an undesired property. This problem could be overcome by adding active electronic circuitry to the sensor, including a local transmitter operating at a frequency other than the frequency used for providing electric power to the sensor and the circuitry. Thereby, the function of wireless power supply should be separated from that of signal transmission and, consequently, the output signal should not be influenced by the position of the sensor. Such a solution has been described by R. Puers, “Linking sensors with telemetry: Impact on the system design”, Proc. 8.sup.th Int. Conf. Solid State Sensors and Actuators, Transducers-95, Stockholm Sweden, Jun. 25-29, 1995, Vol. 1, pp 47-50. However, a drawback of this solution is that it is difficult to miniaturize to the size desired for medical use with a guide wire. Furthermore, wideband systems of this kind are amenable to electromagnetic interference and disturbances.
Thus, there is a need for an improved communication system for communication with a sensor positioned inside a body of a subject for invasive measurement of a physiological variable, said communication system exhibiting reduced sensitivity to the position of the sensor as well as to electromagnetic interference.
U.S. Pat. No. 6,692,446 discloses a method and a device for measuring a physiological variable in a living body, whereby a transmitter is disposed outside of the body to transmit radio frequent energy, and a receiver is disposed outside of the body to receive radio frequent energy. A transponder unit having a sensor sensitive to the physical variable, and a modulator unit for controlling the radio frequent energy absorption of the transponder unit according to a time-sequence representing said physical variable, is introduced into the body. The transmitter sends radio frequent energy to the transponder, and the receiver monitors the radio energy absorption of the transponder unit to determine the time-sequence representing said physical variable. The time-sequence is decoded to interpret it as a measure of the physical variable. Thus, a wireless power supply is provided, and sensitivity to electromagnetic interference is reduced.
However, problems still remain in that the modulator unit and related circuitry is located in a direct proximity to the sensor in the transponder unit disposed in the body. Due to the fact that size requirements on the transponder unit are severe, electronic devices included in the transponder unit must be closely arranged. Moreover, due to these size requirements, it is not possible to use standard electronics in the transponder unit. This has the undesired effect that production of transponder unit electronics becomes rather complex and hence quite expensive.